Oxidation and reduction

A zinc ($$\ce{Zn}$$) atom is oxidised when it loses two electrons to become a soluble zinc ion ($$\ce{Zn^2+}$$).

In covalent bonding, oxidation occurs when an atom gets less than an even share of the electrons it shares in a bond.

The carbon in methane ($$\ce{CH4}$$) is oxidised when methane reacts with oxygen to form carbon dioxide ($$\ce{CO2}$$).

The oxygen atoms exert a strong pull on the electrons in the bonds with carbon in $$\ce{CO2}$$ so that the electrons are more "on the side of the oxygen than the side of the carbon. The carbon atom gets less than an even share of the electrons it shares with the oxygen atoms. Carbon has thus "lost" electrons and is oxidised.

Oxygen atoms exert strong pulls on electrons within molecules. Atoms are thus usually oxidised when they form bonds with oxygen.